Since the metered dose pressurised inhaler was introduced in the mid 1950's, inhalation has become the most widely used route for delivering bronchodilator drugs and steroids to the airways of asthmatic patients. Compared with oral administration of bronchodilators, inhalation offers a rapid onset of action and a low instance of systemic side effects. More recently, inhalation from a pressurised inhaler has been a route selected for the administration of other drugs, e.g., ergotamine, which are not primarily concerned with treatment of a bronchial malady.
The metered dose inhaler is dependent upon the propulsive force of a propellant system used in its manufacture. The propellant generally comprises a mixture of liquified chlorofluorocarbons (CFC's) which are selected to provide the desired vapour pressure and stability of the formulation. Propellants 11, 12 and 114 are the most widely used propellants in aerosol formulations for inhalation administration.
In recent years it has been established that CFC's react with the ozone layer around the earth and contribute towards its depletion. There has been considerable pressure around the world to reduce substantially the use of CFC's, and various Governments have banned the "non-essential" use of CFC's. Such "non-essential" uses include the use of CFC's as refrigerants and blowing agents, but heretofore the use of CFC's in medicines, which contributes to less than 1% of the total use of CFC's, has not been restricted. Nevertheless, in view of the adverse effect of CFC's on the ozone layer it is desirable to seek alternative propellant systems which are suitable for use in inhalation aerosols.
U.S. patent specification No. 4,174,295 discloses aerosol propellant compositions which consist of a mixture of a hydrogen-containing chlorofluorocarbon or fluorocarbon (A), selected from the group consisting of CHClF.sub.2 (Freon 22), CH.sub.2 F.sub.2 (Freon 32) and CF.sub.3 -CH.sub.3 (Freon 143a), with a hydrogen-containing fluorocarbon or chlorofluorocarbon (B) selected from the group consisting of: CH.sub.2 ClF (Freon 31), CClF.sub.2 -CHClF (Freon 123a), CF.sub.3 -CHCl (Freon 124), CHF.sub.2 -CClF.sub.2 (Freon 124a), CHClF-CHF.sub.2 (Freon 133), CF.sub.3 -CH.sub.2 Cl (Freon 133a), CHF.sub.2 -CHF.sub.2 (Freon 134), CF.sub.3 -CH.sub.2 F (Freon 134a), CClF.sub.2 -CH.sub.3 (Freon 142b) and CHF.sub.2 -CH.sub.3 (Freon 152a). The compositions may contain a third component (C) consisting of a saturated hydrocarbon propellant, e.g., n-butane, isobutane, pentane and isopentanes. The propellant compositions comprise 5 to 60% of (A), 5 to 95% of (B) and 0 to 50% of (C) and are said to be suitable for application in the fields of: hair lacquers, anti-perspiration products, perfumes, deodorants for rooms, paints, insecticides, for home cleaning products, for waxes, etc. The compositions may contain dispersing agents and solvents, e.g., methylene chloride, ethanol etc.
It has now been found that 1,1,1,2-tetrafluoroethane has particularly suitable properties for use as a propellant for medicinal aerosol formulations when used in combination with a surface active agent and an adjuvant having a higher polarity than 1,1,1,2-tetrafluoroethane.